In lithographic printing presses, ink and dampening fluid are applied to a lithographic printing plate. The printing plate is mounted onto a plate cylinder which rotates the printing plate past ink and dampening form rollers. The printing plate contains oleophilic and hydrophilic areas. The oleophilic areas are arranged according to the desired image which is to be printed onto media such as paper. The oleophilic areas attract the oil-based ink. Nonprint areas on the printing plate attract the water-based dampening fluid and repel the ink.
In order to achieve satisfactory printing, a proper balance must be achieved between the amount of dampening fluid applied to the printing plate versus the amount of ink that is applied. This is referred to as the ink-water balance. If too much dampening fluid is applied, then the print areas look faded, as the dampening fluid begins to be applied to the oleophilic areas. If too little dampening fluid is applied, then the ink appears in nonink areas.
The inking and dampening fluid are applied to the printing plate with ink and dampening fluid form rollers, respectively. The inking rollers apply ink from a reservoir onto the printing plate, by way of the ink form rollers. Likewise, the dampening form rollers apply dampening fluid from a pan to the printing plate.
The ink form rollers and the dampening form roller may be kept separate from each other. However, in many press configurations, the dampening form roller is bridged to one of the ink form rollers by way of a bridge roller. The bridge roller and the dampening form roller work together to stabilize and maintain the proper ink-water balance.
In one type of prior art dampening system, the bridge roller is rotated by friction by the adjacent dampening form roller and ink form roller. The dampening form roller is generally rotated at the same surface speed as the plate cylinder. However, occasionally, the dampening form roller is slowed down so as to clean hickies off of the plate cylinder. When the dampening form roller changes speed, the speed of the bridge roller also changes, thus affecting the ink-water balance. As the dampening form roller is slowed down, excessive emulsification of the ink and dampening fluid occurs. Excessive emulsification is undesirable, as it degrades print quality. It is therefore desirable to control this mixing by controlling the speed of the bridge roller.